1. Field
The invention relates to preamplifiers and, particularly, to a camera tube preamplifier circuit employing an optical feedback path.
2. Prior Art
One of the basic problems in designing a preamplifier for, for example, the front end of a camera, is how to control the frequency response thereof or, more precisely, how to assess the frequency response. Ideally, the signal current supplied by the image pickup tube (i.e., plumbicon, vidicon, etc.) should be amplified or converted into a voltage without any loss of frequency response. For example, the response should be flat from d.c. to of the order of five or six megahertz (MHz). The problem lies not in making the response flat, but in determining when in fact the response is flat. That is, in order to assess the frequency response of the preamplifier, it must be swept through a range of operating frequencies, which is very difficult to do without disturbing the operating characteristics of the preamplifier, since applying a test sweep to its input inherently distorts the frequency response curve.
The above problem of classic feedback amplifiers for pickup tubes is caused, in part, in the interests of minimizing noise, by the required use of large value load resistors ranging from one-half million to two or three million ohms resistance. The large value load resistors, in turn, have very appreciable self-capacitances as well as distributed capacitances to ground, which cause the feedback to vary with variations in frequency. Furthermore, these parasitic capacitances are not predictable so it is necessary to provide adjustable compensation within the preamplifier. However, in order to make these adjustments, as mentioned above, it is necessary to sweep the preamplifier over the frequency range of interest. Any attempt to do this usually interferes with the characteristics of the preamplifier, thus invalidating the adjustments.
A further problem with classic preamplifiers arises with the use of anti-comet tail type tubes. When discharging highlights, these tubes can generate signal currents as high as 80 microamperes, thus developing 80 volt pulses in a preamplifier using a one million ohm feedback resistance. It is essential that the preamplifier be able to accommodate these pulses without saturating, otherwise excessive recovery times are inevitable. Thus, high voltage supplies are required; and the preamplifier power consumption can become appreciable.
A further disadvantage of classic preamplifiers is the fact that their output d.c. voltage corresponding to zero signal current (black level) is not well defined and varies with temperature. This results in the need for some form of black level clamp to accurately reestablish black level prior to blanking.